Think of a linguist, any linguist. There's only one snag to this game: you're not allowed to pick Noam Chomsky or Steven Pinker. It's not as easy as it sounds; in fact for most people it's more or less impossible. What makes this odder is that everyone feels they are an expert in linguistics - we can all talk - and that the "language instinct" was one of the first places where ideas of innate human nature became respectable again in the 1960s. So here we have a subject of tremendous importance, which goes right into the faculty which, more than any other, distinguishes us as human, yet whose practitioners are mostly anonymous.

Larry Trask should not be anonymous. His short book Language: the Basics, which has been in print since 1995, is the best primer around: it's not just instructive, but written with clarity, verve, and a sense of fun. He hasn't got any sensational theories, but his field, like any other, is extraordinarily interesting when the orthodoxy is well explained: after all, the mainstream of any science is the result of tens of thousands of smart and dedicated scholars refining their knowledge over decades or centuries. This fact is particularly important in linguistics, since it's like teaching, in that everyone feels that a little common sense would solve all its problems.

"Everybody has his own ideas about what is sensible," he says, "and a linguistics based wholly on common sense would be chaotic."

His special language is Basque, which seems natural to the non-linguist: after all, isn't Basque the strangest language on earth? Actually, that's the sort of assertion that drives linguists nuts. All languages have some unique features and Basque is not even unique in being an "isolate", without known relatives. But its isolation is almost complete. Apart from a lost earlier form called Aquitanian, attested by about 500 words in Roman manuscripts, "there is no shred of persuasive evidence", he says, "that Basque is related to any other language at all, living or dead".

The early history of linguistics was a story of discovering language families, and of tracing the resemblances among European languages, so when Basque was discovered, the early linguists were bewildered, and in their bewilderment frequently mistaken. "They reached preposterous conclusions about it, conclusions which have sadly still not disappeared from print," Trask says.

Even though more is understood about Basque today, it remains the most famous isolate in the world, and the one that catches the imagination of anyone when they first become interested in language. It caught Larry Trask's imagination when he was a teenager in a poor family in the Allegheny mountains of western New York state. "I grew up quite literally in the middle of the woods. We have practically no people there, but we have loads of animals: woodchucks, rabbits, possums, raccoons, and gazillions of deer. These days we even have coyotes, which we didn't have when I was a kid. One day when my dad and I were watching a baseball game on TV, a half-grown black bear wandered past the window. We don't see bears very often, because they like to stay deep in the woods."

Growing up in this curiously idyllic isolation. Trask was interested in language, but what he really wanted was to be a scientist. At first, he planned to be an astronomer; but after a few years in school changed his ambition to chemistry. He was only the second member of his family to go to university, but he got his first two degrees as a chemist, and was working towards a PhD when he realised that he would never be terribly good at it: "I still love science, and I still follow it as best I can. But doing chemistry requires one to spend an enormous amount of time preparing to do the work one is interested in: planning experiments, designing the syntheses of required substances, performing the syntheses, purifying the products. And I am not good with my hands, so that lab work took me more time to carry out than a skilful lab chemist would have needed. So I decided to look for another career. I joined the Peace Corps and went off to Turkey to teach chemistry at a university in Ankara, while I pondered things."

His ponderings were interrupted by politics. "One afternoon there was a gunfight outside the window of the lab where I was teaching; the army moved in with tanks and occupied the campus; and I was politely informed by a left-wing organisation that I would be killed if I didn't leave (nothing personal, that's just the way things are when you're American). So I left.

"After that, I came to England for a two-week holiday, which has now gotten a little out of hand: I've been here since 1970."

At first, he taught science in school, and studied linguistics in the evenings at the Polytechnic of Central London. After a year, he was asked to teach the course he had just taken. By then, he had already met in the canteen a Basque, who started to teach him the language. He finally got his PhD in linguistics at the School of Oriental and African Studies: "My thesis was, of course, on Basque. The external examiner turned up for my viva with a blonde on one arm, and a bottle of wine under the other, so I supposed I wasn't going to get a hard time. In fact, things went well.

"Shortly before submitting, I managed to get a job in the linguistics department at Liverpool. I taught there until the 'rationalisation' fad of the late 1980s closed that department and brought me to Sussex, where I've worked since 1988. All this was a good decision. I'm much better at linguistics than I ever was at chemistry."

He's working now on an etymological dictionary of Basque: his web page contains a trenchant denunciation of all the things that people believe about the language: "please note: I do not want to hear about the following: Your latest proof that Basque is related to Iberian/Etruscan/Pictish/Sumerian/ Minoan/Tibetan/Isthmus Zapotec/ Martian. Your discovery that Basque is the secret key to understanding the Ogam inscriptions/the Phaistos disc/ the Easter Island carvings/the Egyptian Book of the Dead/the Qabbala/the prophecies of Nostradamus/your PC manual/the movements of the New York Stock Exchange. Your belief that Basque is the ancestral language of all humankind/a remnant of the speech of lost Atlantis/the language of the vanished civilization of Antarctica/ evidence of visitors from Proxima Centauri. I definitely do not want to hear about these scholarly breakthroughs."

The other thing that really winds him up is Noam Chomsky's reputation among people who don't know the field. Chomsky's basic theory, that the human faculty for language is the result of evolved and innate skills, seems to him indisputable: "I believe that human children are destined to learn language in much the same way that baby birds are destined to sing. I am sympathetic to the proposal that our brains contain areas which are dedicated to language - though I don't want to be dogmatic about this, since the evidence is not yet overwhelming."

He rejects the specific, and controversial theories of Chomsky and his followers - that the brain encodes some kind of "universal grammar", which underlies all the languages that anyone speaks. Trask says any competent linguist can find counter-examples to all the rules Chomskyans propose. "I have no time for Chomskyan theorising and its associated dogmas of 'universal grammar'. This stuff is so much half-baked twaddle, more akin to a religious movement than to a scholarly enterprise. I am confident that our successors will look back on UG as a huge waste of time. I deeply regret the fact that this sludge attracts so much attention outside linguistics, so much so that many non-linguists believe that Chomskyan theory simply is linguistics, that this is what linguistics has to offer, and that UG is now an established piece of truth, beyond criticism or discussion.The truth is entirely otherwise."

Nor does he entirely accept that the language instinct is a single thing, which emerged more or less at once in evolutionary time. There is evidence, he says, that anatomical changes needed to allow our ancestors to speak came hundreds of thousands of years before brain changes which might have given them complex things to say, so that grammar came a long time after babbling.

Trask insists that one of the most striking examples of the failure of common sense is the way people speak about the age of a language, or ask whether one language is older than another. His assertion has been known to wind people into quivering knots on the internet, but it is logically irrefutable.

"Middle English", he says, is, like all other labels given to the language spoken at a particular time, quite arbitrary. It does not represent a thing, or even a historical fact, like the Hundred Years' War. "There are no dividing lines. The speakers in every generation can understand their own parents and their own children without difficulty. In fact, the speakers in every generation could understand the speech of quite a few generations back, and quite a few generations forward, if they could hear it. You are separated from Chaucer's Middle English, and from King Alfred's Old English, by a series of generations all of whom could understand earlier and later speech. Once the time gap becomes suitably large, of course, comprehension becomes increasingly difficult, and it eventually declines to just about zero. But there are no breaks, no discontinuities. Those boundaries, like the 1500 dividing line between Middle English and Early Modern English, are arbitrary."

You can sometimes see a language coming into existence, when creoles -languages originating in extended contact between two language communities - appear. But you can never see a language die, he says. There was never a moment when people stopped talking Middle English.

The same process is much more obvious when we look at the boundaries that languages have in space. A nice example is in Yugoslavia, where everyone over the age of 20 grew up speaking Serbo-Croat, a language now apparently completely replaced by Serbian, Croat and other dialects, even though these are almost exactly the same language, and spoken among the same groups of people. Chinese, he says, provides an example of the opposite sort of classification: the label refers to eight very different languages, all of which are referred to as one language by the Chinese themselves.

Getting an opinion on linguistic questions from Trask is like getting blood from an artery. There's hardly anyone who can make their subject seem more contentious and more fun, which makes for a bitter irony. Illness has robbed him of his voice, so that this interview had to be conducted entirely be email.